1. Field
The invention relates to a method and an apparatus for determining the limpness of sheet material, in particular of bank notes, wherein the sheet material is irradiated with sound waves, the sound waves emanating from the irradiated sheet material are measured, and the determination of the limpness of the sheet material is effected on the basis of the measured sound waves.
2. Related Art
Such a system is known for example from the U.S. Pat. No. 5,922,959. Here the sound portion reflected by a bank note exposed to sound waves is measured. The lesser sound reflected by the checked bank note, the greater the degree of limpness of the bank note. Furthermore, there is described that alternatively also the degree of sound wave transmission can be determined.
This known proceeding, however, has the disadvantage that the degree of limpness of the bank notes can be determined in an only insufficient manner. Other influence quantities, such as the degree of soiling of the bank note, folds or crumple, also lead to a changed absorption of sound, so that the information about the limpness of the bank note paper required in the known system cannot be obtained in a clear fashion by means of the known method.
On these premises, it is the problem of the present invention to provide a method and an apparatus for determining the limpness of sheet material, that have a higher accuracy.